1. Field of the Invention
The present invention relates to an optical hybrid module comprising a plurality of optical devices integrated on an optical waveguide substrate. More particularly, the present invention relates to an optical hybrid module and a manufacturing method thereof for reducing optical crosstalk between a plurality of optical devices caused by reflections and leakage of light within the optical hybrid module.
2. Description of the Related Art
There has been an increased need for bi-directional optical transmission and reception systems in the recent past. As a result, there also has been an increase in the needs for optical transmitter/receiver modules. The manufacture of such an optical transmitter/receiver module normally requires a plurality of optical devices, such as an optical source, a photodetector and an optical waveguide. Although optical modules can be manufactured by independent assembly of the plurality of optical devices, respectively, it is less competitive in the view of cost and size of products. Therefore, it is preferable to construct an integrated type optical transmitter/receiver module.
With regard to techniques for achieving the integration of the optical module, a great deal of use has been conducted with a hybrid integrated optical module. This type of module is one in which an active device, such as an optical transmitter/receiver, and a passive device, such as a wavelength division multiplexer, constitute hybrid integrated devices on an optical waveguide substrate.
FIG. 1 is a view illustrating one exemplary construction of an optical hybrid module of the prior art. This prior art optical hybrid module comprises a multi-layer thin film filter 11 vertically inserted in a substrate 12. The multi-layer thin film filter 11 serves to separate two lights having different wavelengths, wherein the lights enter through optical fiber 13 and are emitted from optical source 14.
Referring to FIG. 1, certain optical signals having a specific wavelength λ1 enter through the optical fiber 13, and these optical signals reach the multi-layer thin film filter 11 located at the opposite side to the optical fiber 13 by being guided through a first optical waveguide formed at the substrate 12. The multi-layer thin film filter 11 is adapted to reflect only a specific wavelength λ2. If the wavelength λ1 of the light entering through the optical fiber 13 is different from that of the reflection wavelength λ2, the light with a wavelength λ1 is transmitted through the multi-layer thin film filter 11, so as to reach photodetector 15, (such as a photodiode).
On the other hand, the light emitted from an optical source, such as a laser diode, which light has the wavelength λ2 difference from the light that already entered through the optical fiber 13, enters a second optical waveguide and is guided there through. The guided light, however, is reflected by the multi-layer thin film filter 11 because the wavelength λ2 is the reflection wavelength of the filter 11, thereby exiting to the outside through the optical fiber 13.
The prior art optical hybrid module described above has a disadvantage in that light produced from a light emitting device is leaked or reflected, thereby ineffectively transmitting to a light receiving device. This inefficiency causes crosstalk of optical signals.
FIG. 2 illustrates another exemplary construction of a prior art optical hybrid module, which was created with the goal of eliminating the above disadvantage of crosstalk. In accordance with the illustrated construction, the optical module is of a type wherein an optical waveguide core portion 22 and cladding portion 23 are formed on a substrate 21, and a light emitting device and a light receiving device 26 are disposed thereon. FIG. 2 also enlarged the view regarding where the light receiving device 26 is mounted. In FIG. 2, reference numeral 24 indicates a portion provided with electrical wiring and a solder layer for mounting the light receiving device 26, and other hatched portions are those on which a light blocking layer 25 is formed. The light blocking layer 25 is made of a metal film. As the light blocking layer 25 is provided around the light emitting device and light receiving device 26, it is possible to prevent light from entering the light receiving device 26 from lower and lateral portions of the light receiving device 26.
The optical module of the prior art shown in FIG. 2 has a vertical surface at a region where it is optically coupled with the light receiving device, that is, at a region including the optical waveguide core portion. But, it is actually very difficult to form the light blocking layer at such a vertical surface. Thus, the optical module as shown in FIG. 2 above is a hypothetical structure that, when considered from a manufacturing viewpoint, is virtually impossible to realize.